basic fluid dynamics of very small droplets. The main challenge in studying the dynamics of tiny droplets is the very fast motions, which can also occur at very small scales. This is sometimes overcome by using stroboscopic techniques, but the finer details may not be repeatable enough. A novel ultra-high-speed video camera, developed by our collaborator Prof Etoh at Kinki University in Osaka, Japan, is used in our study. The camera allows image capture at rates as high as 1 million frames per second (fps).
Figure 1 shows a video sequence taken with this camera at 100,000 fps, where an inkjet droplet is being squeezed out of a 50 mm glass nozzle. During manufacturing with inkjet printers there may be as many as 1000 such droplets being squeezed out per second. In this sequence we note that a tiny satellite droplet has formed behind the primary droplet. Such satellite droplets must be avoided in many applications, as their random deposition can contaminate the primary patterns being printed.
Our imaging of droplets and bubbles at these very short time-scales and small length-scales has produced many other surprises. Figure 2 shows a related phenomenon which occurs during the pinch-off of a large water droplet. Here the large-amplitude oscillations of a satellite drop can produce micro-jetting. The micro-jet is produced when capillary waves focus and collide at the bottom of the drop. The resulting droplets are only 30 mm in diameter and emerge at a high velocity of about 10 m/s. It might be possible to exploit this phenomenon for improving piezo-driven printing and generating sub-micron droplets.
Figure 3 presents another very rapid free-surface phenomenon, which we are able to observe now with our new camera system. It shows the pinching-off of a bubble from a vertical nozzle. It is perhaps surprising that the pinch-off of a bubble in water is much faster than the pinch-off of a water drop in air. The underlying dynamics are also quite different, as is evident by comparing Figures 2 and 3. Surface tension drives the pinch-off of a droplet, while the bubble pinches off by inertial focusing and surface tension becomes irrelevant during the final stages of bubble pinch-off. The close-up images in Figure 3 show that for a more viscous liquid, a fine filament of air is stretched out in the neck region, during the final stage of the pinch-off. Capillary instability breaks this thin thread of air into numerous tiny bubbles, which are about 5 mm in diameter or smaller. While such micro-bubbles may be detrimental in some processes, they may also serve as nucleation sites for chemical reactions or boiling. The dynamics of the micro air thread itself is of interest and is critically dependent on the surface properties, such as the presence of surfactants. The study of these micro surface phenomena might therefore give new insights into the behavior of rapidly deforming free surfaces in complex liquids and are bound to produce even more surprises.
